The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A typical ceramic heater generally includes a ceramic substrate and a resistive heating element either embedded within or secured to an exterior surface of the ceramic substrate. Heat generated by the resistive heating element can be rapidly transferred to a target object disposed proximate the ceramic substrate because of the excellent heat conductivity of ceramic materials.
Ceramic materials, however, are known to be difficult to bond to metallic materials due to poor wettability of ceramic materials and metallic materials. Moreover, the difference in coefficient of thermal expansion between the ceramic material and the metallic material is significant and thus a bond between the ceramic material and the metallic material is difficult to maintain.
Conventionally, a power terminal is attached to the ceramic substrate in one of two methods. In the first method, a metal foil is brazed to a part of the resistive heating element to form a terminal pad, followed by brazing the power terminal to the metal foil. The metal foil and the power terminal are brazed to the ceramic substrate in a non-heating zone to avoid generation of thermal stress at high temperatures during operation. Creating a non-heating zone solely for the purpose of securing the power terminal, however, does not seem practical and economical, given the trend of compact designs in many areas including the ceramic heaters.
The second method involves drilling a hole in the ceramic substrate to expose a part of the resistive heating element and placing the power terminal within the hole, followed by filling the hole with an active brazing alloy to secure the power terminal to the resistive heating element and the ceramic substrate. Unlike the first method, the power terminal of the second method is secured to the ceramic substrate in a heating zone. Again, the incompatible thermal expansion among the ceramic materials, active brazing alloy and metallic materials causes thermal stress at high temperatures at the interface between the ceramic substrate and the active brazing alloy, resulting in cracks in the ceramic substrate proximate the hole.